The present invention relates to a light-conducting unit, which is used as a surface light source of a back-light for a liquid crystal display or an outdoor signboard, or as an interior or exterior lighting device. The light-conducting unit of the present invention can be used as a substitute for a conventional light-conducting plate, and can reduce the weight of a light-conducting unit and in turn a surface light source, since it has a hollow light-conducting space therein.
As shown in FIG. 1, a conventional surface light source uses a solid light-conducting plate 10 to emit light, which is received from a spot light source or a linear light source, that is, a lamp 4 in FIG. 1. The light-conducting plate is usually made of a polymer material such as an acrylic resin. The light from the light source enters in the light-conducting plate through the light-incident edge (light-incident side face), and is allowed to emit from one of the major surfaces of the light-conducting plate while the light propagates through the plate from the light-incident edge towards the remote edge of the plate opposing the light-incident edge. Thus, the surface light emission is achieved.
The propagation of the light through the light-conducting plate utilizes total internal reflection of light at the interface between air and the light-conducting plate due to the difference of the refractive index between the air and the light-conducting plate (polymer material). Such a light-conducting plate is an essential part of a back-lighting surface light source for a liquid crystal display. Such a light-conducting plate and a back-light comprising the same are disclosed, for example, in Japanese Laid Open Patent Application A-11-142845.
However, when a solid light-conducting plate is used, it is difficult to reduce the weight of the surface light source. For example, the area of the light-conducting plate (light-emitting area) and also the thickness of the surface light source are being increased with the enlargement of the liquid crystal display screen. Thus, the weight increase of the light-conducting plate makes it very difficult to reduce the weight of the surface light source and, in turn, the weight of the liquid crystal display. Even in the case of small-sized liquid crystal displays, it is highly desired to reduce the weight of the surface light sources in the case of portable terminals such as mobile PCs, cellular phones, PHS, etc
Accordingly, it has been proposed to use a hollow light-conducting unit instead of the solid light-conducting plate. For example, a hollow light-conducting unit is known, which comprises a light-emitting plate and a back plate which are arranged in parallel with each other to form a hollow light-conducting space, and two prismatic films which are placed on the respective surfaces of the light-emitting plate and the back plate facing inside of the light-conducting space.
The prismatic film is a well known optical element and means a light-transmitting film one major surface of which is flat, while the other major surface of which carries a plurality of prisms arranged in parallel with each other in the lengthwise direction (so-called parallel prisms). Such a light-transmitting film is usually placed between the light-conducting plate and an object to be illuminated (e.g. a liquid crystal display panel, etc.).
To allow the light to emit from the light-conducting unit, in general, a linear light source (e.g. a fluorescent tube, etc.) is placed along the opening which is formed at one end of the above light-conducting space, and the light is irradiated in the light-conducting space through the opening. Such an opening is formed by arranging the above pair of the prismatic films substantially in parallel with each other with leaving a gap between them. The pair of the prismatic films are usually arranged with allowing their flat surfaces to be in parallel with each other.
Such a light-conducting unit has a first opening formed at one end of the unit, and a second opening formed at the other end of the unit opposing the one end (the farthest end from the one end), and two light sources are placed at the respective openings to irradiate light into the light-conducting space. Since the pair of the prismatic films are arranged in parallel with each other, the height of the light-conducting space (in the thickness direction), that is, the size perpendicular to the light-emitting surface, does not substantially change from the first opening at the one end to the second opening at the farthest end from the one end.
One embodiment of such a hollow light-conducting unit, and a surface light source comprising the light-conducting unit and a light source is proposed by the present inventor in JP-A-6-180552. Prior to the invention of JP-A-6-180552, another embodiment of the light-conducting unit was known.
In the other embodiment of the light-conducting unit, (i) the arranging direction of the prisms of the prismatic film on the light-emitting plate is in parallel to the direction of light from the light source, and the prismatic surface carrying the parallel prisms is arranged to face the outside of the light-conducting space. Furthermore, (ii) the direction of the prisms of the prismatic film on the back plate is also in parallel with the direction of light from the light source, and the prismatic surface is arranged to face the outside of the light-conducting space. In the case of the surface light source comprising such a light-conducting unit, the illumination of light emitted from the light-emitting surface tends to decrease as the distance from the light source increases. Thus, an additional optical element is necessary to rectify such the decrease of the illumination. Such an additional optical is a so-called extractor, which has sawtooth-form reflective projections the distances of which increase as the distance from the light source increases. Such an extractor is usually arranged on the flat surface of the prismatic film on the back plate side.
The present inventor proposed, in JP-A-6-180552, an improved light-conducting unit to provide an inexpensive light-conducting unit and a surface light source which does not require any extractor and thus can be very easily designed and produced, and a surface light source.
That is, the proposed hollow light-conducting unit comprises a pair of prismatic films which are fixed in respective positions so that they are arranged in the specific three-dimensional configuration, wherein (I) the prismatic surface of one prismatic film, which functions as a light-emitting plane, is arranged, so that the direction of the prisms of this prismatic film is perpendicular to the direction of light from the light source, and the prismatic surface faces the inside of the light-conducting space. Furthermore, (II) the other prismatic film is arranged so that the direction of the prisms of the other prismatic film is in parallel with the direction of light from the light source, and the prismatic surface faces the inside of the light-conducting space. Thereby, the illumination on the light-emitting surface is made uniform without the use of an extractor.
With this above light-conducting unit, a part of the light emitted from the light source is reflected by the prismatic film, while the remaining part of the light passes through the prismatic film on the light-emitting side and is emitted from the light-emitting surface (leakage of light). The light, which is reflected by the prismatic film, is emitted from the light-emitting surface or propagated from one end near the light source to the farthest end from the light source by being repeatedly ed and transmitted by the prismatic films in the light-conducting space. That is, the balance between the leakage of light and the propagation of light is important. If the light-leaking effect is too high, the amount of light to be propagated decreases so that the uniformity of light emission tends to deteriorate. The increase of the light-leaking effect leads to the increase of the illumination brightness.
Accordingly, the optimization of the construction of the light-conducting unit, for example, the optimization of the three-dimensional configuration of the two prismatic films and the combination of the shapes of the prisms, or the selection and use of optical elements such as extractors, is necessary to balance the leakage of light and the propagation of light and to attain the desired brightness and also the uniformity of emitted light, in accordance with the application, size and the like of the light-conducting unit, that is, the surface light source.
As described above, the hollow light-conducting comprising a pair of the prismatic films which are arranged in parallel with each other and fixed in the specific configuration can reduce the weight of the unit itself, the surface light source and an apparatus comprising the surface light source (e.g. a liquid crystal display, etc.).
However, it is difficult to produce the above-described conventional light-conducting unit having a relatively small thickness. It is highly desired to further reduce the thickness of the liquid crystal device with the enlargement of the liquid crystal display screen. Furthermore, it is desired to further decrease the thickness of the above-described portable terminals.
To satisfy such requirements, it is necessary to decrease the thickness of the light-conducting unit, that is, the thickness (height) of the light-conducting space, to 30 mm or less, and preferably, 20 mm or less.
The reduction of the thickness of the light-conducting unit means the decrease of the height of the light-conducting space, that is, the size of the space which is in perpendicular to the propagation direction of the light. On the other hand, the area of the light-emitting surface, that is, the propagation distance of light, does not decrease significantly in connection with the decrease of the thickness of the unit. After the above-described proposal of the present inventor in JP-A-6-180552, further investigations have been made. As a result, it has been found that a large amount of light may leak near the edge at which the light source is placed, or almost all the amount of light may be propagated to the farthest end without leakage, when the height of the light-conducting space is relatively low. Therefore, it is necessary to more effectively balance the light-leaking effect and the light-propagation effect than ever, in order to easily realize the reduction of the thickness of the unit However, the use of additional optical elements such as an extractor may make the structure of unit complicated, and make it difficult to reduce the weight of the unit.
The present invention provides a light-weight light-conducting unit which can effectively balance the light-leaking effect and the light-propagation effect of the prismatic films forming the light-conducting space, and satisfy the requirements to reduce the weight of the unit.
According to the first aspect of the present invention, there is provided a light-conducting unit comprising the first prismatic film and the second prismatic film, each having two major surfaces, one of which is a prismatic surface, and the other of which is a flat surface carrying no prism, said prismatic surface carrying a plurality of prisms which are arranged substantially in parallel with each other along the lengthwise direction of the prisms, wherein said two prismatic films are arranged so that the first edges of said prismatic films are positioned substantially in parallel with each other leaving a gap between them to form an opening, a light-conducting space, which is in continuous from said opening, between said prismatic films, and the height (the size in the thickness direction) of said light-conducting space does not substantially increase from said opening to the remote edges opposing said opening, and the major surface of said first prismatic film, which faces outside said light-conducting space, is a light-emitting surface to emit light which is propagated from said opening into said light-conducting space, characterized in that said prismatic surface of said first prismatic film is placed facing outside of said light-conducting space, and the direction of said prisms of said first prismatic film is not parallel with the direction of said light.
According to the second aspect of the present invention, there is provided a light-conducting unit comprising the first prismatic film and the second prismatic film, each having two major surfaces, one of which is a prismatic surface, and the other of which is a flat surface carrying no prism, said prismatic surface carrying a plurality of prisms which are arranged substantially in parallel with each other along the lengthwise direction of the prisms, wherein said two prismatic films are arranged so that the first edges of said prismatic films are positioned substantially in parallel with each other with a gap between them to form an opening, a light-conducting space, which is in continuous from said opening, between said prismatic films, and the height (the size in the thickness direction) of said light-conducting space does not substantially increase from said opening to the remote edges opposing said opening, and the major surface of said first prismatic film, which faces outside said light-conducting space, is a light-emitting surface to emit light which is propagated from said opening into said light-conducting space, characterized in that said prismatic surface of said first prismatic film is placed facing outside of said light-conducting space and the direction of said prisms of said first prismatic film is parallel with the incident direction of said light, and that the arranging direction of said prism of said second prismatic film is not parallel to the direction of said light.
According to the third aspect of the present invention, there is provided a light-conducting unit comprising a first prismatic film and a second prismatic film, each having two major surfaces, one of which is a prismatic surface, and the other of which is a flat surface carrying no prism, said prismatic surface carrying a plurality of prisms which are arranged substantially parallel with each other along the lengthwise direction of the prisms, wherein said two prismatic films are arranged so that the first edges of said prismatic films are arranged substantially parallel with each other leaving a gap between them to form an opening and a light-conducting space, which is in continuous from said opening, formed between said prismatic films, and the major surface of said first prismatic film, which faces outside said light-conducting space, is a light-emitting surface to emit light which is propagated from said opening into said light-conducting space, characterized in that said prismatic surface of said first prismatic film is placed facing outside of said light-conducting space, the direction of said prisms of said first prismatic film is parallel with the direction of said light, said prismatic surface of said second prismatic film is placed facing outside of said light-conducting space, and also the direction of said prism of said second prismatic film is parallel with the direction of said light, and that said two prismatic films are arranged so that the height (the size in the thickness direction) of said light-conducting space decreases from said opening to the remote edges opposing said opening.
According to the fourth aspect of the present invention, there is provided a light-conducting unit comprising a first prismatic film and a second prismatic film, each having two major surfaces, one of which is a prismatic surface, and the other of which is a flat surface carrying no prism, said prismatic surface carrying a plurality of prisms which are arranged substantially in parallel with each other along the lengthwise direction of the prisms, wherein said two prismatic films are arranged so that the first edges of said prismatic films are positioned substantially parallel with each other leaving a gap between them to form an opening, a light-conducting space, which is continuous from said opening, between said prismatic films, and the height (the size in the thickness direction) of said light-conducting space does not substantially increase from said opening to the remote edges opposing said opening, and the major surface of said first prismatic film, which faces outside said light-conducting space, is a light-emitting surface to emit light which is propagated from said opening into said light-conducting space, characterized in that said prismatic surface of said first prismatic film is placed facing inside of said light-conducting space, while said prismatic surface of said second prismatic film is placed with facing outside of said light-conducting space.
According to the fifth aspect of the present invention, there is provided a light-conducting unit comprising a first prismatic film and a second prismatic film, each having two major surfaces, one of which is a prismatic surface, and the other of which is a flat surface carrying no prism, said prismatic surface carrying a plurality of prisms which are arranged substantially parallel with each other along the lengthwise direction of the prisms, wherein said two prismatic films are arranged so that the first edges of said prismatic films are positioned substantially parallel with each other leaving a gap between them to form an opening, a light-conducting space, which is continuous from said opening, between said prismatic films, and the height (the size in the thickness direction) of said light-conducting space does not substantially increase from said opening to the remote edges opposing said opening, and the major surface of said first prismatic film, which faces outside said light-conducting space, is a light-emitting surface to emit light which is propagated from said opening into said light-conducting space, characterized in that said prismatic surface of said first prismatic film is placed with facing inside of said light-conducting space, and also said prismatic surface of said second prismatic film is placed facing inside of said light-conducting space, and that the direction of said prisms of said first prismatic film is not perpendicular to the incident direction of said light.
According to the sixth aspect of the present invention, there is provided a light-conducting unit comprising a first prismatic film and a second prismatic film, each having two major surfaces, one of which is a prismatic surface, and the other of which is a flat surface carrying no prism, said prismatic surface carrying a plurality of prisms which are arranged substantially parallel with each other along the lengthwise direction of the prisms, wherein said two prismatic films are arranged so that the first edges of said prismatic films are positioned substantially parallel with each other leaving a gap between them to form an opening, a light-conducting space, which is continuous from said opening, between said prismatic films, and the height (the size in the thickness direction) of said light-conducting space does not substantially increase from said opening to the remote edges opposing said opening, and the major surface of said first prismatic film, which faces outside said light-conducting space, is a light-emitting surface to emit light which is propagated from said opening into said light-conducting space, characterized in that said prismatic surface of said first prismatic film is placed facing inside of said light-conducting space, and said prismatic surface of said second prismatic film is placed with facing inside of said light-conducting space, and the direction of said prisms of said first prismatic film is perpendicular to said direction of said light, and that the apex angle of said prism of said first prismatic film is smaller than that of said prism of said second prismatic film.